Low bias current/temperature compensation current mirror for linear power amplifier

ABSTRACT

A power amplifier circuit is disclosed, whose power efficiency is optimized by operating its stages in substantially close to a Class B mode by reducing the quiescent current during low driver signal levels. As the driver signal amplitude increases, the amplifier is dynamically biased to operate in a Class AB mode. A further enhancement to the power amplifier circuit includes a temperature compensation circuit to adjust the bias of the amplifier so as to stabilize the performance over a wide temperature range.

This application claims the benefit of U.S. Provisional Application No.60/148,099, filed Aug. 10, 1999.

FIELD OF THE INVENTION

This invention relates to amplifier circuitry and, more particularly, toa low bias current and temperature compensation current mirror for usein a linear power amplifier.

BACKGROUND OF THE INVENTION

Power amplifiers are categorized into several classes of operation. Someof these classes include Class-A, Class-B and Class-AB. A Class A poweramplifier is defined as an amplifier with or without negative feedback,and in its ideal case is characterized with the greatest fidelity infaithfully amplifying an input signal with the least distortion. Itconducts output current throughout 100% of the input signal waveform. Inother words it exhibits a conduction angle of exactly 2π radians. Inmost cases it exhibits the greatest gain of all power amplifier classes.Another characteristic of the Class A amplifier is that its DC biaspoint is generally selected to be at ½ the transistor's peak currentcapability and ½ its peak voltage capability. It is however the leastefficient of all the classes of amplifiers, in as much that in idealcases the power delivered to the load is typically only 50% of the D.C.power used.

A Class AB amplifier is defined in the ideal case as a power amplifierthat has an output current flow for more than half, but less than all,of the input cycle. In other words it exhibits a conduction anglebetween π and 2π radians. See, e.g., Gilbilisco, Stan, Ed. AmnateurRadio Encyclopedia, TAB Books, 1994. Another characteristic of a ClassAB amplifier is that it is generally biased at less than ½ of thetransistor's peak current capability. The advantages of a Class ABamplifier include improved high power efficiency over a Class A typeamplifier and improved efficiency at low drive levels. The drawbacks ofClass AB power amplifiers include the generation of an output signalwhich is not an exact linear reproduction of the input waveform, lowergain than that of a Class A type, continuous current drain, and lowerefficiency compared to other amplifier types.

An ideal Class B amplifier is defined as an amplifier that has outputcurrent flow for ½ the cycle of the input signal wave form. In otherwords it exhibits a conduction angle of π radians. The advantages of aClass B amplifier are improved high power efficiency over Class A and ABtype amplifiers and improved efficiency at low drive levels. Thedrawbacks of a Class B amplifier include even higher distortion andlower gain compared to Class A and AB amplifiers.

Class A, AB and B amplifiers are typically used in the transmitters ofcellular mobile terminals. The Class selected is often dictated by thecommunications standard employed by the terminal. Typically a GSM typehandset will utilize a Class B amplifier stage as the final amplifier ofthe transmitter. This is because the GMSK standard employed in a GSMhandset embeds the voice or data being transmitted in the phase angle ofthe signal, which is somemtimes referred to as a constant envelopesignal. Such signals are more tolerant to amplitude distortion duringthe amplification and transmission process. One benefit of using a ClassB amplifier in these handsets is that it results in longer battery lifeand thus longer talk times.

In an NADC or CDMA cellular mobile terminal the final amplifier in thetransmitter chain is typically a Class AB type amplifier. This isbecause the data or voice being transmitted is encoded in both theamplitude and phase of the signal. This results in a signal with anon-constant envelope, requiring a transmitter having a minimal amountof both amplitude and phase distortion.

Although Class AB operation improves power efficiency at the cost ofsome linearity of signal amplification, it has become the amplifierclass of choice for non-constant envelope mobile cellular and PCStransmitters. Unlike the Class B type amplifier, the Class AB requires aquiescent current bias. And although it exhibits better efficiency atlow drive levels than a Class A type amplifier, the optimum low driveefficiency is limited by the linearity requirement under higher drive.In general, as power efficiency improves under high drive, linearitywill suffer. The inverse relationship of efficiency at high drive, lowquiescent bias, high efficiency at low drive and the need for high drivelinearity makes the selection of a quiescent bias point a criticaldesign parameter. The operation of the Bipolar or FET transistors at lowquiescent bias points also exposes the amplifier to greater variabilityin performance at both low and high operating temperatures.

Ideally, a power amplifier for use in mobile terminal equipment such ascellular or PCS communication devices employing a non-constant envelopemodulation scheme should amplify the input signal linearly with minimaldistortion of the signal and with optimum efficiency across a wide rangeof drive levels.

In practice, balancing linearity and efficiency across all driveconditions is difficult. Accordingly, a need exists for an improvedbiasing arrangement that dynamically adjusts the operating mode of thepower amplifier as a function of drive signal, temperature or otherrelevant factors. Such a biasing arrangement would both eliminateexcessive power dissipation in the output stages at low drive levels andminimize distortion over a broad range of drive levels.

SUMMARY OF THE INVENTION

In one aspect, the present invention comprises an amplifier biasingcircuit. The biasing circuit is configured to set a bias point tooperate the amplifier in a deep Class AB mode or an almost Class B mode(i.e., a mode substantially close to a Class B mode) during low drivelevel (quiescent current) conditions. The depth of the Class AB mode isbounded by the worst case allowable distortion at those drive levels.When the drive level increases, the biasing circuit dynamically adjuststhe bias point in such a manner that the amplifier operates in a ClassAB mode and allows the amplifier to accommodate high drive levels, againbounded by the worst case allowable distortion. Other parameters may beused alternatively or in addition to drive level to adjust theamplification mode. For example, distortion levels may be detected, bymeans that would be apparent to those of ordinary skill in this field,and may be used to adjust the bias point or other parameters in order todynamically modify the amplification mode.

In another aspect, the present invention comprises a circuit to achievetemperature compensation. The temperature compensation circuit isconfigured to adjust the bias point of the amplifier over a temperaturerange. In a yet another aspect, the invention is directed to amonolithic integrated circuit comprising the biasing circuit.

In a further aspect, the invention is a method of dynamically adjustingthe bias points of a multi-stage amplifying circuit, comprising thesteps of detecting the drive level at a driver stage of the amplifyingcircuit and dynamically adjusting the bias points of a single stage orall of the stages of the entire multi-stage amplifying circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be more readily apparent from the following detaileddescription of the preferred embodiments, where like numerals designatelike parts, in which,

FIG. 1 is a block schematic of a preferred embodiment of the presentinvention;

FIG. 2 is a detailed schematic of a preferred embodiment of the presentinvention;

FIG. 3A shows the characteristics of the V_(tss) versus peak AC swing;

FIG. 3B shows the output power P_(OUT), Gain, and efficiencycharacteristics of a temperature compensated power amplifier deviceconstructed according to the principles of the present invention, andcontrasted with uncompensated devices; and

FIG. 4 is a characteristic of five devices in accordance with thepresent invention wherein the bias current I_(dq) is shown astemperature varies between −30° C. and +110° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a linear power amplifier including the novel dynamicbias features of the present invention. The amplifier includes apre-amplifier stage 110, a driver amplifier stage 120, and an outputamplifier stage 130. In a preferred embodiment, the output of the driveramplifier stage 120 is coupled to a driver output detector 140. A biasadjustor circuit 150 in accordance with the present invention is coupledto the driver ouptut detector 140. Additionally, the output of the biasadjustor circuit 150 is coupled to the three-stage amplifier circuit viaa coupling circuit comprising resistors R′ and R″. Preferably resistorsR′ and R″ are each 2KΩ. Temperature compensation circuit 160 is coupledto the bias adjustor circuit 150.

Each of the three stages 110, 120, 130 is preferably comprised of adepletion mode Gallium Arsenide (GaAs) Field Effect Transistor (FET)gain amplifier. In general, the depletion mode FET currentcharacteristics are such that it draws maximum current (I_(dss)) whenits gate-to-source voltage (V_(gs)) is zero, and minimum current (zero)when its gate-to-source voltage is negative Vp. The various modes ofoperation, i.e., Class A, Class B and Class AB, are achieved by applyinga suitable amount of gate-to-source voltage. Thus, the modes ofoperation can be defined by the percentage of the maximum current drawnat each stage.

In a preferred embodiment, the pre-amplifier stage 110 is designed tooperate in a Class A mode and primarily functions as a gain stage. Thedriver amplifier 120 is configured to operate in a Class AB mode. Theoutput amplifier 130 is configured to operate in a Class AB mode orsubstantially close to a Class B mode.

Referring now to FIG. 2, in a preferred embodiment, the driver outputdetector circuit 140 comprises diode D1, resistor R1, capacitor C1,filter capacitor C2 and resistive divider comprising resistors R2 andR3.

The dynamic bias adjustor circuit 150 preferably comprises a pair ofField Effect Transistors (FET) Q3 and Q4 considered a reference FieldEffect Transistor. Q3 is bias-coupled to the driver output detectorcircuit 140. The drain of Q3 is preferably coupled to a current limitingresistor R_(o). In one embodiment, the source of Q3 is coupled toground. In an alternative embodiment, the source of Q3 is coupled toground via a resistor of suitable size.

A source-follower circuit, preferably comprising FET Q5 is coupled toboth Q3 and Q4 (the reference FET). The source follower is provided witha reference voltage, V_(refB), applied at the gate of Q5. A plurality ofdiodes and resistors form a level-shifting circuit 170. In a preferredembodiment, four diodes D2, D3, D4, D5 and resistor Rs form alevel-shifting circuit 170 to produce a combined voltage level drop ofabout 2.6V. A FET Q6 with its gate and source tied together acts as acurrent source establishing the current through diodes D2, D3, D4 andD5. It is operatively coupled to the level-shifting circuit 170 asshown.

The temperature compensation circuit 160 preferably comprises an FET Q7and a plurality of diodes connected to its gate. In a preferredembodiment, two diodes, D6 and D7 are used.

OPERATION OF THE CIRCUIT

One aspect of the present invention is that a master-slave relationshipis established between the driver stage 120 and the output stage 130 ofthe multi-stage power amplifier of FIG. 1. As the driver stage 120detects a need to generate a larger AC output voltage, it instructsitself and the other stages in the power amplifier to alter theiroperating modes from a deep Class AB or B mode to a Class AB mode. Thus,the multi-stage power amplifier dynamically adjusts its bias point todeliver optimum efficiency across a broad range of drive levelconditions. It has been found that this approach additionally reducesthe required idle (quiescent) current by about 60%. Further, thisinvention minimizes or extends the gain compression characteristicstypically exhibited by fixed Class AB or Class B mode power amplifiers.Moreover, the present invention controls and stabilizes the bias pointof a multi-stage power amplifier over a wide temperature range of −30°C. to +110° C.

The driver output detector circuit 140 detects the alternating voltage(AC) signal swing at the driver output and generates a negative DirectCurrent (DC) voltage proportional to the peak AC swing V_(ac) (i.e., thevoltage range from peak-to-peak). The capacitor C1 filters out thefundamental frequency component of V_(ac) and the resistive dividercomprising resistors R2 and R3 determines the slope of the relationshipbetween the peak AC voltage V_(ac) and the DC output voltage (V_(tss)),as well as the maximum value that V_(tss) can reach. FIG. 3A shows thecharacteristics of the V_(tss) versus peak AC power (P_(in)).

The voltage V_(tss) obtained at the driver output detector is applied tothe gate of Q3. This gate voltage controls current I₁ through ResistorR_(o). Change in the current I₁ results in a change in the voltageV_(refB) applied at the source follower Q5. The output of the sourcefollower Q5 is level-shifted by the level-shifting circuit 170 to theappropriate gate-to-source voltage V_(gs). V_(gs) is used to set theoperating mode of the three stages 110, 120 and 130 of the poweramplifier.

At no drive or very low drive level conditions at the input of thepre-amplifier stage 110, the peak AC signal at the output of the driverstage 120 is very small or zero. Therefore, voltage V_(tss) is also verysmall or zero. This voltage, V_(tss), is applied to the gate of thetransistor Q3. This forces Q3 to draw a maximum current, causing thevoltage V_(refB) to drop. The drop in V_(refB) forces the gate-to-sourcevoltage, V_(gs), for all the stages, to become more negative, therebymaking them draw less current. This pushes the operating mode of theoutput stage of the power amplifier towards a Class B or substantiallyclose to a Class B mode. Operating each power amplifier stage at lowcurrents increases the power-added efficiency of the amplifier andreduces the overall DC operating current at very low drive levels.

As the drive level to the pre-amplifier stage 110 increases, thedriver's peak AC output voltage V_(ac) increases. When the peak ACoutput voltage V_(ac) exceeds approximately 0.65 V (which is the forwardbias voltage drop for a Gallium Arsenide diode), the driver outputdetector 140 will begin to generate a negative voltage proportional tothe peak AC output voltage. As the drive is increased, this voltagebecomes more and more negative and approaches the maximum level of DCvoltage, which is determined by the resistor R2. As V_(tss) increases,the transistor Q3 draws less current, causing an increase in the voltageV_(refB). This increase in voltage in turn increases the voltage Vgs,thereby shifting the bias point of the amplifier stages toward a ClassAB mode. Thus, an efficient operation with minimal spectral distortionis achieved under large signal conditions.

A further feature of the circuit presented here is temperaturecompensation circuit 160. The plurality of diodes D6 and D7 areconfigured to bias the transistor Q7 close to pinch off (Vp), i.e., thepoint at which Q7 permits current flow. A negative voltage, supplied bya series resistor Rref connected to Vss is also used to bias thetransistor Q7. The operating point of transistor Q7 is similar to thatof the output stage 130 of the amplifier. Diodes D6 and D7 are selectedsuch that they establish the required rate of change in the voltage dropacross them versus temperature. Transistor Q7 and resistor Rref areselected such that the required slope in temperature compensation isachieved.

As temperature increases, the voltage drop across the pair of diodesdecreases causing the transistor Q7 to draw more current. This creates avoltage drop in V_(refB) which is level shifted to Vg causing it also tobecome more negative, in turn causing the three stages of the amplifierto draw less current and maintain the decreased current level. Anopposite result occurs when the temperature decreases. FIG. 3B shows theoutput power P_(OUT), Gain, and efficiency characteristics of atemperature-compensated power amplifier constructed according to theprinciples of the present invention. FIG. 4 illustrates the variation ofthe bias current I_(dq) versus temperature for five devices inaccordance with the present invention, wherein the bias current I_(dq)is shown as temperature varies between −30° C. and +110° C. The figureillustrates that the amplifier's bias point can be controlled using thetemperature compensation circuit described above. Also shown forcomparison are plots for two uncompensated circuits in which the biaspoint changes linearly with temperature.

The embodiments described herein are merely illustrative and notintended to limit the scope of the invention. One skilled in the art maymake various changes, rearrangements and modifications withoutsubstantially departing from the principles of the invention. Forexample, a bias adjustment in accordance with the present invention canbe achieved by detecting increases or decreases in distortion of theoutput signal (e.g., by comparing the input signal to the outputsignal). To reduce AM-AM distortion of a non-constant envelope signal,the instantaneous output AC swing can be detected and the bias point canbe dynamically adjusted in order to minimize gain compression (i.e.,change from Class B mode to Class AB mode). This would result in reducedAM-AM distortion during the high instantaneous voltage peaks.Alternatively, in another example the output AC voltage swing can bedetected to monitor substantial changes in output loading conditions.Variations in load conditions can result in substantial increases inpeak output AC voltage swings that can be excessive and cause permanentdamage to the output stage. These peak AC swings can be detected and anappropriate bias point change can be applied in order to either turn offthe power amplifier or substantially reduce the overall power amplifiergain (i.e., change from Class AB mode to Class B mode).

Also, certain portions of the described circuitry can be modified toinclude discrete components, whereas the remaining portion can be etchedinto a monolithic integrated circuit. Additionally, the materialsdescribed herein may be changed. Accordingly, all such deviations anddepartures should be interpreted to be within the spirit and scope ofthe following claims.

What is claimed is:
 1. An amplifying circuit, comprising: an amplifyingdevice; and a dynamic bias adjustor connected to said amplifying device,which detects a drive level of said amplifying device, and is configuredto operate said amplifying device in a Class B mode or substantiallyclose to a Class B mode during low drive level conditions, and in aClass AB mode during high drive level conditions.
 2. The amplifyingcircuit as in claim 1, further comprising a temperature compensationcircuit to stabilize a bias point of said amplifying device over atemperature range.
 3. The amplifying circuit as in claim 2, wherein thetemperature range is between −30° C. and +110° C.
 4. The amplifyingcircuit of claim 1, wherein said amplifying device comprises apre-amplifier stage, a drive stage, and an output stage, and theamplifying circuit further comprises a signal detector connected betweenthe output of said drive stage and said dynamic bias adjustor.
 5. Theamplifying circuit of claim 4, wherein said dynamic bias adjustor biasesat least one of said pre-amplifier stage, said drive stage, and saidoutput stage to operate in a Class B mode or substantially close to aClass B mode, when an output signal level detected by said signaldetector is below a predetermined level.
 6. The amplifying circuit ofclaim 5, wherein said dynamic bias adjustor biases at least one of saidpre-amplifier stage, said drive stage, and said output stage to operatein a Class AB mode, when the output signal level detected by said signaldetector is above a predetermined level.
 7. The amplifying circuit ofclaim 1, further comprising: a signal detector connected to an output ofsaid amplifying device; and wherein said dynamic bias adjustor furthercomprises a differential amplifier, connected to said signal detector,and a source follower, connected between said differential amplifier andan input of said amplifying device; whereby said signal detectorproduces a signal that is proportional to the output of said amplifyingdevice, said differential amplifier amplifies said signal, and theoutput of said source follower adjusts an operating point of saidamplifying device according to the amplified signal.
 8. The amplifyingcircuit of claim 2, wherein said temperature compensation circuitcomprises: a transistor having a control input and two outputs, whereinone of said outputs is connected to ground and the other is connected tosaid dynamic bias adjustor; and at least one solid-state element havingan impedance that varies according to its temperature, connected at oneend to the control input of said transistor and at another end toground; a reference resistor connected at one end to the junction of thesolid-state element and the control input of said transistor and capableof receiving at another end a negative supply voltage; wherein saidtransistor outputs a temperature-dependent signal to said dynamic biasadjustor that is proportional to the temperature of said solid-stateelement, and said dynamic bias adjustor alters the operating point ofsaid amplifying device to compensate for changes in thetemperature-dependent signal output from said transistor.
 9. A method ofbiasing an amplifying circuit comprising a pre-amplifier stage, a driverstage and an output stage, the method comprising the steps of: detectinga driver output alternating voltage at the driver stage; applying to adynamic bias adjuster a voltage proportional to the driver outputvoltage; and applying an output voltage of the dynamic bias adjuster tobias the stages of the amplifying circuit whereby at least one of thestages of the amplifying circuit operates in a Class B mode orsubstantially close to a Class B when the driver output current is at orbelow a predetermined level.
 10. The method of claim 9, furthercomprising the step of changing the bias voltage applied to at least oneof the stages of the amplifying circuit in order to operate said atleast one of the stages of the amplifying circuit in a Class AB modewhen the driver output current is at or above a predetermined level. 11.The method of claim 9, further comprising the step of stabilizing thebias point of at least one of the stages to account for changes intemperature.
 12. A method of biasing an amplifying device, comprisingthe steps of monitoring a signal level at an output of the amplifyingdevice; producing a first signal proportional to the signal level; anddynamically biasing, as a function of the first signal, the amplifyingdevice to operate in a Class B mode or substantially close to a Class Bmode when the signal level is low, and in a Class AB mode when thesignal level is high.
 13. The method of claim 12, further comprising thesteps of: producing a second signal proportional to the temperature ofthe amplifying device; and dynamically biasing, as a function of thesecond signal, the amplifying device so that its operating point issubstantially stable over a temperature range.
 14. The method of claim13, wherein the temperature range is between −30+ C. and +110° C. 15.The method of claim 12, further comprising the steps of: amplifying thefirst signal, level-shifting the first signal, and using thelevel-shifted signal to alter the operating point of the amplifyingdevice.
 16. The method of claim 13, wherein the step of producing asecond signal comprises the steps of: establishing a current-flowthrough a solid-state element having an impedance that varies accordingto its temperature, thereby creating a temperature-dependent voltagesignal; applying the temperature-dependent voltage signal to atransistor, whereby the output signal of the transistor also variesaccording to temperature; and level-shifting the second signal.